Helmet mounted displays consist of a display which is attached to a helmet, so as to be visible to a person e.g. a pilot, wearing the helmet. Current helmet displays generally rely on high resolution CRTS (cathode ray tubes), the images on the CRT screen being imaged by conventional optical elements and eyepieces, and as a consequence are bulky and heavy.
It is known to mount the CRT remote from the helmet and conduct an image of the CRT screen to the helmet via a light path comprising a bundle of optical fibres, the helmet incorporating conventional optics to produce an image which the user can see. However, the bundle of optical fibres required to implement this is relatively bulky and expensive. It is also known to replace conventional optics with holographic elements, thereby reducing weight and bulk: however, such holographic elements suffer unacceptable degrees of optical dispersion when used in conjunction with CRTs.
It is known from UK patent 1423947 to provide a head-up display for an aircraft in which the windscreen of the aircraft comprises a holographic reflector. The pilot views a liquid crystal display via this holographic reflector and a relay lens. This prior art is bulky and is unsuitable for mounting directly on a helmet.
It is known from U.S. Pat. No. 4,361,384 to provide a helmet-mounted display arrangement in which an image is viewed via an optical path which includes reflection by a holographic reflector disposed on the visor of a helmet worn by the viewer. In this prior art the image is produced by means of a relatively low-power flat panel display and an image intensifier.
The present invention provides a display assembly suitable for mounting on a helmet in which the display assembly is formed by a spacial light modulator capable of being illuminated by a source of laser light, an optical system comprising at least one holographic element for forming an image of the said display, and an optical path between the spatial light modulator and the source of laser light, the optical path comprising a single optical fibre.
The use of a spatial light modulator illuminated by a laser as the display enables holographic elements to be used in the optical system.
The image producing system may include a holographic collimator. The spatial light modulator may comprise an electrically addressable spatial light modulator. The spatial light modulator may be illuminated by laser light. The optical fibre may be provided with a mode scrambler.